1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including at least one of these functions, and more particularly to, an image forming apparatus capable of detecting a density of an image and an image forming method.
2. Description of the Related Art
There have been known image forming apparatuses using electrophotography in which an image is formed using toner. The known image forming apparatuses, such as copiers, facsimile machines, printers, or multi-functional systems having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a development device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In recent years, such image forming apparatuses have spread throughout printing industries, and demand for higher output speed and higher image quality has been increasing rapidly. The demand for higher image quality includes a strong demand for density uniformity within a page, i.e., uniformity in density of an image formed on a recording medium, such as a sheet of paper. Uniform density within a page has become one criterion that users use to select an image forming apparatus. It is therefore important to minimize uneven density in a page.
Uneven density is caused by various elements, such as uneven charging of the image baring member due to non-uniform charging, uneven exposure by the exposure device, uneven rotation or uneven sensitivity of the image bearing member such as a photosensitive member, uneven resistance of a developer bearing member such as a development roller, uneven charging of toner, and uneven transfer by a transfer roller, for example. Uneven density attributed to, among other things, the uneven rotation or uneven sensitivity of the image bearing member particularly occurs at relatively short intervals, and thus periodically occurs within a page and is easily noticeable, which is often the cause of a complaint. It is therefore particularly important to suppress uneven density attributed to the uneven rotation or uneven sensitivity of the image bearing member.
The above-described varieties of uneven density will now be described in detail.
Uneven density attributed to the uneven rotation of the image bearing member will be described first. In the electrophotographic image forming apparatus, an electric field generated by the potential difference between the developer bearing member and the image bearing member causes toner to adhere to an outer circumferential surface of the image bearing member, to form an image. The uneven rotation of the image baring member, therefore, changes a development gap between the image baring member and the developer bearing member, resulting in a change in the electric field and thus a change in image density.
Meanwhile, uneven density attributed to the uneven sensitivity of the image bearing member occurs as follows. If the sensitivity of the image bearing member to exposure varies over time or due to a change in ambient conditions, even when the image bearing member is exposed at a constant level, there will be a difference in a bright portion potential, which is a potential of the image bearing member after exposure. As a result, the electric field fluctuates and hence the density changes, resulting in uneven density. Reducing the changes in the sensitivity of the image bearing member by using a highly accurate manufacturing method leads to a higher cost which complicates efforts to reduce the manufacturing cost.
In order to correct uneven density, process parameters including a charging bias, a development bias, exposure conditions may be changed based on a profile of uneven density which changes according to the rotational period of the image bearing member and uneven sensitivity. For example, image patterns for detection of uneven density are formed to create correction data for each process parameter.
According to this method, the development bias is modulated in accordance with the rotational period of the image bearing member, for example. More specifically, in this method, a rotational position detector for detecting the rotational position of the image bearing member and a density detector for detecting the density of the image are employed. According to this method, uneven density detected by the density detector is divided by the rotational period of the image bearing member, and the development bias is periodically changed with the use of the signal of the rotational position detector as a trigger, such that the change of the electric field due to elements such as the uneven rotation of the image bearing member is canceled to stabilize the electric field and thereby suppress the detected uneven density.
In this method, not only the development bias, but also the charging bias may be modulated.
Meanwhile, uneven density attributed to the uneven sensitivity of the image bearing member occurs as follows.
The sensitivity of the toner adhesion amount to the electric field changes in accordance with the image density. That is, the sensitivity of the image bearing member to the electric field changes in accordance with the image density, thereby changing the sensitivity of the image bearing member. More specifically, in a shadow portion having a relatively high density, such as a solid image portion having a relatively large toner adhesion amount, a development potential (i.e., a potential difference between the development bias and the bright portion potential) is dominant. By contrast, in a halftone or highlighted image portion smaller in the toner adhesion amount than the shadow portion, a background potential (i.e., a potential difference between the development bias and the dark portion potential) is dominant. Herein, the dark portion potential refers to the potential of a non-exposed portion of the image bearing member.
If a parameter such as the development bias is controlled to correct uneven density in the shadow portion, therefore, the effect of the control is not achieved in the halftone or highlighted image portion, and uneven density is increased.
Furthermore, there is known a method in which uneven density appearing periodically as a streak in an image may be comprehensively reduced in an electrophotographic or electrostatic-recording image forming apparatus such as in JP-H09-062042-A. In this method, the image forming apparatus includes a first fluctuation data storage device which stores periodical density fluctuation data of the image density in advance and a first controller which controls an image forming condition on the basis of the density fluctuation data. According to this method, the first fluctuation data storage device stores density fluctuation data corresponding to at least one rotational period of the developer bearing member, and the first controller controls one of the charging voltage, the exposure light amount, the development voltage, and the transfer voltage to correct the density in accordance with the rotational period of the image bearing member.
Uneven density may be suppressed by another method focusing not on the rotational period of the image bearing member but on the rotational period of the developer bearing member such as in JP-2000-098675-A (JP-3825184-B). The method changes the development bias in accordance with the rotational period of the development roller to reduce uneven density in the image occurring cyclically with the rotational period of the development roller. More specifically, the method detects uneven density in image patterns formed on the image bearing member, and performs phase matching between the detected uneven density information and the rotation of the development roller, thereby controlling the development bias.
If the control target is limited to the development bias, however, it is highly possible that the image density correction works in the solid image but not in the halftone image.
In such a case, in addition to changing the development bias, the charging bias may be changed in accordance with the rotational period of the development roller. As described above, the method in which the image density in the halftone image is effectively corrected by changing the development bias and the charging bias in accordance with the rotational period of the development roller may be applied to correction of the image density in the halftone image by changing the development bias and the charging bias in accordance with the rotational period of the image bearing member.
If the uneven sensitivity of the image bearing member as mentioned above is caused by its own characteristics, the uneven sensitivity occurs with the rotational period of the image bearing member. Thus, changing the development bias and the charging bias in accordance with the rotational period of the image bearing member may suppress the uneven sensitivity.
Uneven density attributed to the uneven rotation and the uneven sensitivity of rotators, i.e., the image bearing member and the developer bearing member may be effectively corrected using the correction data for correcting image forming conditions such as the development bias and the charging bias cyclically with the rotational period of the image bearing member and the development bearing member.
In order to perform the correction, it is necessary to apply accurately the correction data created to correspond to the rotational period of the rotators at the phase at which the uneven density is canceled. For example, in a case in which a toner image detector reads the image patterns created for detection of the uneven density, and the correction data for correcting parameters such as the development bias and the charging bias to suppress the uneven density in the image patterns is created, it is preferable to obtain the following time information. That is, the time information refers to a time period from an operating point of the control of the development device and the charging device, that is, a position at which the development bias and the charging bias are applied, to the time at which the toner image detector detects the image patterns.
This is because even when the phase of the rotators is detected, if the layout distance between the operating point of the control and the toner image detector is not an integral multiple of the rotational period of the rotators, it is difficult to apply accurately the correction data in such a manner as to suppress the uneven density. The time information may be obtained by dividing the layout distance by the rotation speed of the rotators if the layout distance and the phase of the rotators are known.
Even when the time information is obtained, in reality, it is difficult to apply accurately the correction data in such a manner as to suppress the uneven density. As a result, desirable correction of the uneven density is not performed using the correction data.
The difference between a design value of the layout distance and the actual layout distance causes difficulty in correction of the uneven density. Taking the design value of the layout distance into consideration is not enough to accurately match the phase of the correction data with the phase of the uneven density. In an actual machine, due to accumulation of manufacturing tolerances and installation errors of parts, and parts variations, there is a discrepancy between the design layout distance and the target distance. As a result, there is a discrepancy between the time information obtained from the design value of the layout distance and the actual time information so that the position of generation of the uneven density on the rotator and the phase of the correction data, for example, a control table, do not correspond to each other, resulting in insufficient correction effects.
Another cause is a shift in the control table due to a delay in the response from a high-voltage power pack serving as a power source to apply the development bias and the charging bias. Because there is a delay in the response from the high-voltage power pack that outputs the development bias and the charging bias, the phase of the uneven density and the phase of the control table do not coincide with each other, hence resulting in insufficient correction effects. In general, when the rotational period is short, that is, when the frequency is high, the delay in the response is long.
Furthermore, without operating the actual machine, the substantial control operating point of the development bias and the charging bias cannot be determined. In other words, the substantial control operating point of the development bias cannot be determined when using a development device using a multi-stage roller development system and the like. The substantial control operating point of the charging bias also cannot be determined when using a scorotron charging method using a grid electrode. These control operating points are known only when the actual machine is operated. Therefore, it is difficult to know the layout distance based on the design control operating points. In view of the above, it is necessary to obtain the control points somehow. The same is true for a single-stage development roller and a charging method using a roller.
If only taking the layout position into account, it is difficult to accurately apply the correction data for the uneven density which is made to correspond to the rotational period of the rotators in such a manner as to suppress the uneven density.
In view of the above, there is demand for an image forming apparatus that can apply accurately correction data for uneven density corresponding to a rotational period of a rotator to suppress the uneven density.
Such a configuration is proposed, for example, in JP-2006-267486-A.
Conventionally, there is known a single-beam optical scanning device employed in, but not limited to, an image forming apparatus, such as a digital copier or a laser printer, in which a recording speed is increased by increasing a rotation speed of a polygon mirror serving as a deflector.
In view of the above, there is demand for a device capable of minimizing operational load.